Advice! Subbing with heating pads

Hey everyone, ive done plenty of research on substitution, and have successfully substituted twice. When i subbed i put a bottle in my underwear around the gooch area, from trial and error it seemed the temp stayed pretty accurate. My question is, with an upcoming drug test and me living in a cold climate this fall/winter should i assume thats enough. I hear alot of ppl using heat pads but im wondering if they always get it at body temp? Do it ever go over the natural body temperature? I would use one but dont want to submit a sample too warm and im worried without it maybe it will lose heat with colder weather. Advice?